Solution-processed red CPL-OLEDs enabled by an exciplex-forming host and chiral helicene dopant

Abstract

A strategy for the fabrication of efficient solution-processed red circularly polarized luminescent organic light-emitting diodes (CPL-OLEDs) has been proposed. Two mCP-derived carbazole materials, Cz2Cz and 2Cz2Cz, were synthesized and utilized as electron donors for exploring exciplex formation with an electron acceptor, PO-T2T. PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blends exhibit high photoluminescence quantum yields (PLQYs) of 26–27% and effective thermally activated delayed fluorescence (TADF) behavior, endowing OLED devices with electroluminescence centered at 506 nm and 503 nm, V_on of 4.8 V and 4.6 V, and EQE_max of 6.38% and 7.30%, respectively. The well-overlapped emission of the exciplex-forming blends and the absorption of the newly designed chiral helicene–perylene diimide emitter 3 facilitate an efficient Förster resonance energy transfer (FRET) process. Molecule 3 with excellent molecular rigidity exhibits red emission (630 nm) and strong CPL characteristics (|g_lum| ≈ 10⁻³) in solution. The incorporation of 3 as a dopant dispersed in the exciplex-forming co-host matrix affords red-emitting CPL-OLEDs with an EQE_max of 1.41% and |g_EL| of up to 1.1 × 10⁻³. These results demonstrate that the synergistic combination of TADF-enabling exciplex-based co-hosts and chiral helicene emitters to achieve CPL-OLEDs is a versatile approach for advanced chiral optoelectronic applications.

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Introduction

Circularly polarized luminescence (CPL) materials are attracting attention for application in 3D displays, optical sensing, and quantum communication.^1–5 Incorporating CPL emitters into organic light-emitting diodes (OLEDs) enables the fabrication of next-generation devices with improved stereoscopic effects and potential application in chiral photonics and data encryption.^6–8 An important parameter that indicates the CPL characteristic is the luminescence dissymmetric factor, e.g. g_lum for photoluminescence (PL) and g_EL for electroluminescence (EL), which is determined as g = 2(I_L − I_R)/(I_L + I_R), where I_L and I_R refer to left-handed and right-handed polarized light intensity, respectively. The g value is between −2 and +2, in which a negative g is right-handed, and a positive g is left-handed. Conventionally, CPL can be feasibly achieved by employing chiral moieties to perturb π-extended achiral chromophores, particularly those incorporating C₂-symmetric chiral units such as chiral binaphthyl and 1,2-diaminocyclohexane derivatives.^9–11 In recent years, helicenes have emerged as exceptional scaffolds for developing emissive materials with strong CPL activity owing to their inherent helical chirality and superior photophysical properties.^12–15 Based on the helicity rule proposed by Cahn, Ingold, and Prelog,¹⁶ a left-handed helix is designated as M (minus), and a right-handed helix is designated as P (plus). There is a general relationship between the absolute configuration and chiroptical properties. For instance, it has long been known that (M)-helicenes are levorotatory, while (P)-helicenes are dextrorotatory.^17,18

CPL-OLEDs have emerged as a prominent research focus due to their facile fabrication, simple structure, low cost, and tunable optical and chiral optical physical properties.¹⁹ In 2015, Zuo's group reported highly efficient CPL-OLEDs with a |g_EL| of 2.6 × 10⁻³, utilizing chiral iridium (Ir) complexes as emitters.²⁰ In 2017, Huang's group further advanced this field by achieving a |g_EL| of 3 × 10⁻³ using chiral Ir isocyanide complexes, representing the highest reported g_EL for CPL-OLEDs based on Ir complexes to date.²¹ Nevertheless, their broader application is limited by the high cost and scarcity of precious metals. To address this, in 2020, Zheng's group reported four pairs of chiral fluorescence enantiomers derived from 1,2-diaminocyclohexane, achieving a g_lum of −7.2 × 10⁻⁴ with an exceptional PL quantum yield (PLQY) of 93.14%. The corresponding sky-blue CPL-OLEDs exhibited a g_EL of −8.3 × 10⁻⁴ and an external quantum efficiency (EQE) of 5.5%.¹¹ In 2021, Yang's group integrated thermally activated delayed fluorescence (TADF) and CPL properties into a naphthalimide derivative, producing orange CPL-OLEDs with a g_EL of −2.4 × 10⁻³ and an impressive EQE of 23.7%.²² Also in 2021, Favereau's group introduced a series of helicene derivatives, functionalized with ethynyl and trimethylsilyl groups. These derivatives exhibited a remarkable CPL performance (g_lum ∼ 10⁻²) with symmetrical structures, demonstrating twice the performance of their asymmetrical counterparts.²³ However, the integration of CPL-active materials into the OLED architecture remains challenging. Although many CPL materials exhibit strong chiroptical properties in solution or solid films, their performance often deteriorates in device settings due to issues such as molecular aggregation, exciton quenching, and limited energy transfer compatibility.²⁴ To mitigate aggregation-induced energy quenching in OLEDs, the design of host–guest systems has emerged as a practical strategy. The incorporation of co-host systems, often realized as exciplexes, into emissive layer (EML) design has been widely adopted to regulate the exciton dynamics and enhance the device performance, given that high-efficiency OLEDs not only require balanced charge mobility but also effective exciton utilization. Although conventional wide-bandgap hosts are commonly used, co-host systems can facilitate more efficient energy transfer between the host and dopant under electrical excitation. Exciplexes are formed through physical blending an electron donor (D) and an acceptor (A), typically via co-evaporation or spin-coating. Upon photo-excitation, the excited state of the D (or A) interacts with its ground-state counterpart, forming a through-space intermolecular charge-transfer (CT) excited state, commonly referred to exciplex (Fig. 1). This CT-state features complete spatial separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals, which is conducive to achieve TADF behavior,^25,26 offering a reverse intersystem crossing (RISC) pathway to harvest electro-generated triplet excitons. Beyond their roles as efficient EMLs, exciplex-forming blends also serve as promising co-hosts for fluorescent dopants.^27–33 Notably, the well-overlapping emission of the exciplex-forming blend and the absorption of the fluorescent guest facilitates an efficient Förster resonance energy transfer (FRET) process from the exciplex co-host to the guest, while simultaneously lowering the charge injection barrier and reducing the device turn-on voltage (V_on). Additionally, exciplex-forming co-hosts enable spectral tunability toward the near-infrared region and provide smoother, more balanced carrier transport.^27,28,34 These collective features make exciplex-forming co-hosts a highly effective platform for high efficiency OLEDs. Herein, we introduce a new class of EML materials, comprising two N,N′-dicarbazolyl-3,5-benzene (mCP)-centered donors, Cz2Cz and 2Cz2Cz (Fig. 1), which was developed as donors for exploring the formation of new exciplex-forming co-host systems with the acceptor 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) and a novel chiral helicene derivative as a CPL emitter (3_P/M). Cz2Cz and 2Cz2Cz were modified by introducing one and two carbazolyl (Cz) groups at the C1-position of carbazole of mCP, respectively. The introduction of Cz as substitution(s) of mCP results in a twisted molecular geometry that effectively suppresses aggregation formation, while also preserving the hole transport character owing to the low reorganization energy of Cz. In addition, the twisted conformation impedes the effective extension of the π-conjugation length, thereby sustaining a higher triplet energy level of Cz2Cz and 2Cz2Cz, which can prevent back-electron transfer after exciplex formation. Furthermore, additional Cz substitution(s) could increase the D–A interactions, leading to a higher propensity for exciplex formation. The PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blended films prepared via the solution process exhibit sky-blue TADF characteristics with a PLQY of 26% and 27%, respectively. The devices employing the PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blends as the exciplex-forming EML exhibit the EL maximum (λ_EL) of 506 nm and 503 nm with EQE_max of 6.38% and 7.30%, respectively. Importantly, the strong spectral overlap between the EL and absorption of helicene-based CPL emitter 3 ensures efficient FRET, leading to the development of red CPL-OLEDs with λ_EL of 626 nm and EQE_max of 1.41%. Notably, the CPL-OLEDs exhibit g_EL ∼ 10⁻³, demonstrating the potential of using exciplex-forming blends as the co-hosts doped with a tailor-made helicene emitter as the EML for the development of high-efficiency CPL-OLEDs.

Fig. 1 (a) Chemical structures of Cz2Cz and 2Cz2Cz donors, PO-T2T acceptor, and CPL-emitter 3. (b) Schematic representation of exciplex formation and the FRET mechanism.

Results and discussion

The synthesis of the new mCP-centered donors, Cz2Cz and 2Cz2Cz, is depicted in Scheme 1a. Cz2Cz and 2Cz2Cz were synthesized starting from the precursor 1,9′-bicarbazole 1Cz-Cz, which was prepared by the C–H bond activation method.³⁵ Subsequently, Cz2Cz and 2Cz2Cz were synthesized via palladium-catalyzed Buchwald–Hartwig coupling reaction and copper-catalyzed Ullmann coupling reaction, with the yield of 92% and 57%, respectively. The details of the synthetic routes and structural characterization are provided in SI. The molecular structures and synthesis of CPL emitter 3 are shown in Scheme 1b. The molecular design of 3 integrates the chiral properties of helicene and the luminescent efficiency of perylene diimide (PDI). The introduction of alkyl chains on the periphery of PDI not only increases the solubility of 3 but also mitigates the propensity of aggregation-induced quenching typically observed in PDI derivatives. Compound 3 (including P- and M-form) was synthesized via the palladium-catalyzed Buchwald–Hartwig coupling reaction of racemic dichlorohelicene 1³⁶ with the corresponding PDI-carbazole 2,³⁷ affording a yield of 68%. The room-temperature ¹H NMR spectrum of 3 in CDCl₃ showed poorly resolved broadened signals, rendering its detailed structure analysis difficult. Inspection of its structure suggests that the observed undesirable spectral features may be ascribed to the hindered rotations along six bonds connecting individual fragments of the molecule (see Fig. S5 and the discussion therein). Racemic 3 was separated by semipreparative HPLC on a ChiralArt Cellulose-SC column (YMC) for further characterization and application. The assignment of helicity was achieved through independent experiments, as discussed in a later section.

Scheme 1 Synthetic routes to (a) mCP-centered donors Cz2Cz and 2Cz2Cz and (b) CPL molecule rac-3 (only P enantiomer shown).

The photophysical properties of two mCP-centered donor molecules, Cz2Cz and 2Cz2Cz, were examined. As shown in Fig. 2a, the two donors exhibit nearly indistinguishable absorption (Abs) and emission characteristics. The main electronic Abs peaks of Cz2Cz and 2Cz2Cz appear at 292 nm, corresponding to π–π* transitions, while the weaker Abs features observed at 336 and 338 nm are attributed to the n–π* transitions of carbazole moieties.³³ Consistently, Cz2Cz and 2Cz2Cz display fluorescence (Fl) peaks centered at around 360 nm. The triplet energy (T₁) was estimated from the onset of the phosphorescence spectrum in toluene at 77 K. 2Cz2Cz exhibits a slightly shallower triplet energy (3.04 eV) compared to that (3.06 eV) of Cz2Cz, indicating the effect of one additional Cz substitution on the T₁ level. Nevertheless, these high T₁ energy levels (>3.00 eV) indicate limited π-conjugation extension due to the highly twisted molecular conformations induced by Cz substitution(s) on the mCP core. These high T₁ levels are advantageous for suppressing back electron transfer processes, thereby supporting their effective role in forming exciplex systems. Moreover, in neat films, both donors exhibit red-shifted and broadened emission profiles, which are attributed to aggregation effects (Fig. 2b). Notably, 2Cz2Cz shows a more structureless emission spectrum, suggesting a relatively stronger aggregation-induced effect compared to Cz2Cz, likely due to its symmetric molecular structure, which increases the propensity for intermolecular interactions. The corresponding photophysical properties are summarized in Table 1.

Fig. 2 UV-vis absorption (Abs) and fluorescence (Fl) spectra at room temperature, and phosphorescence (Phos) spectra at 77 K of Cz2Cz and 2Cz2Cz in (a) toluene solution and (b) a neat film.

Table 1 Physical characteristics of Cz2Cz2 and 2Cz2Cz

Compound	λ ^solAbs (nm)	λ Abs (nm)	λ Fl (nm)	λ Fl (nm)	E T (eV)	HOMO^d (eV)	LUMO^e (eV)	E g (eV)
a Measured in toluene solution (10^−5 M). b Measured in a neat film. c Estimated from the onset of the phosphorescence spectra at 77 K in toluene. d HOMO level was calculated from oxidation potential with reference to the HOMO of ferrocene. e Calculated from the difference between the HOMO and corresponding optical bandgap. LUMO levels were obtained from the onset of the Abs spectrum (Eg). f Estimated from the onset of the UV–vis Abs curves in toluene.
Cz2Cz	289, 336	243, 297, 329, 344	357, 367	382, 405, 427	3.06	−5.66	−2.19	3.47
2Cz2Cz	289, 338	244, 298, 329, 345	358, 371	402	3.04	−5.47	−2.05	3.42

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Given that the energy levels of the donor play a crucial role in determining the exciplex emission, the electrochemical properties of Cz2Cz and 2Cz2Cz were investigated by cyclic voltammetry (CV), with the results presented in Fig. S17 and Table 1. Cz2Cz and 2Cz2Cz exhibit quasi-reversible oxidation processes with oxidation potentials at 0.86 and 0.67 V vs. Fc/Fc⁺,³⁸ respectively. The corresponding HOMO energy levels of Cz2Cz and 2Cz2Cz are calculated as −5.66 and −5.47 eV, respectively, based on the onset potential of the first oxidation process.³⁸ The shallower HOMO level of 2Cz2Cz can be reasonably ascribed to the higher number of electron-donating Cz groups. This finding is consistent with density functional theory (DFT) calculations performed at the B3LYP/6-31G+(d) level (Fig. S18), which show that the HOMO is delocalized and degenerate over the peripheral Cz units.

Fortunately, crystals of 2Cz2Cz suitable for X-ray diffraction (XRD) analysis were successfully obtained via the slow diffusion of orthogonal solvents (dichloromethane (DCM)/methanol). The corresponding crystallographic data are summarized in Table S1. Notably, 2Cz2Cz displays a rigid molecular configuration that enables favorable intermolecular stacking, as illustrated in Fig. S20a and S20b. Multiple intermolecular interactions with distances shorter than 3.5 Å can be identified, implying π–π interactions between the alternating molecular stacking arrangement.

Fig. 3a–d depict the UV-vis Abs, Fl, and CPL spectra of 3 at room temperature in toluene solution. Its corresponding photophysical properties are summarized in Table 2. Referring to Fig. 3a, the observed fine-structured profile of the UV-vis Abs, and Fl spectra originate from the rigidity of its structure, which minimizes the vibration modes and related non-radiative pathways. Regarding the 0–0 electronic transition, the absorption and emission of 3 are centered at 534 nm and 535 nm, respectively, which can be attributed to the n–π* coupling with π–π* transition from its PDI carbazole pendant fragment. The subtle Stokes shift suggests that both the central helicene backbone and the peripheral chromophore PDI exhibit high rigidity, implying the high PLQY of 3. The PLQY of 3 in DCM solution was determined to be 11% by using an integrating sphere. Notably, the fine structure of its PL spectrum exhibits a concentration-dependent shift, as shown in Fig. S22. Time-resolved PL (TrPL) experiments were conducted to probe the relaxation process of 3 in toluene solution and the corresponding data is summarized in Table 2. The emission of 3 behaves as a conventional fluorescent emitter, with decay dynamics that can be well described by a single lifetime on the nanosecond timescale (Fig. 3b). The electrochemical properties of 3 are shown in Fig. S17b and Table 2. Its LUMO is −3.61 eV, calculated from the reduction potential with reference to the HOMO of ferrocene. The HOMO level is −5.86 eV, determined from the onset of the Abs spectrum (E_g) as the optical energy gap and LUMO energy. As shown in Fig. 3c, the absolute configuration P is indicated for enantiomer (+)-3 in the exciton-coupled electronic circular dichroism (ECD) spectrum in the region corresponding to the PDI units (around 525 nm), showing first a positive (higher wavelength), and then a negative (shorter wavelength) Cotton effect³⁹ (resulting from clockwise screw sense of the two interacting chromophores). Also, the positive ECD band at 300–400 nm can most likely be ascribed to the P-helical carbohelicene scaffold. Given that the ECD spectrum of (+)-3 could not be easily fitted with a time-dependent DFT (TD-DFT) simulated one, we decided to disperse any possible doubts about its absolute configuration by resolving the precursor, dichlorohelicene 1, into enantiomers (HPLC conditions and chromatograms are shown in SI as Fig. S13–S15), and then performing the synthesis of 3 starting from a single enantiomer following identical reaction conditions. The comparison of the measured ECD spectra with the TD-DFT calculated one was unambiguous in this case (Fig. S16) and assigns M-helicity to the fast-eluting enantiomer and P-helicity to the slow-eluting enantiomer. Given that the fast-eluting enantiomer (M)-1 provided the slow-eluting PDI-derivative (−)-3, we assign P-helicity to the fast-eluting enantiomer (+)-3 and M-helicity to the slow-eluting enantiomer (−)-3. Similar to the ECD spectra, perfect mirror images of the CPL spectra (Fig. 3d) were observed for the (+) and (−) enantiomers of 3. Interestingly, inverted CPL signals were observed within the same emission region (530–750 nm), with the latter part of the spectrum displaying a broader and slightly stronger CPL signal, suggesting the potential for effective CPL amplification through concentration-induced aggregation.⁴⁰ The excellent CPL properties and high PLQY of 3 reveal its great underlying potential for CPL-OLED application.

Fig. 3 (a) UV-vis Abs, and Fl spectra of 3 (10⁻⁵ M in toluene), (b) time-resolved PL (TrPL) spectra of 3 (10⁻⁵ M in toluene), (c) Abs and ECD spectra of 3 (10⁻⁵ M in toluene), and (d) Fl and CPL spectra of 3 (10⁻⁵ M in toluene).

Table 2 Physical characteristics of 3

Compound	λ ^solAbs (nm)	λ Abs (nm)	λ Fl (nm)	λ Fl (nm)	HOMO (eV)	LUMO^c (eV)	E g (eV)	τ (ns)	g PL
a Measured in toluene solution (10^−5 M). b Measured in a neat film. c LUMO level was calculated from reduction potential with reference to the HOMO of ferrocene. HOMO levels were obtained from the onset of the Abs spectrum (Eg). d Estimated from the onset of the UV-vis Abs curves in toluene. e Lifetime was obtained from TrPL measurements.
3	460, 490, 527	237, 493, 534	536, 578, 620	630	−5.86	−3.61	2.25	1.09, 5.45	+5.1 × 10^−4/-4.3 × 10^−4

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The photophysical properties of the D:A blends were examined to explore the exciplex formation. The Abs spectra of the PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blend films exhibit the individual D and A components without new spectral features, indicating the absence of ground-state electronic coupling between D and A. In contrast, the PL spectra of the PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blend films exhibit a pronounced red shift and spectral broadening, with emission peaks centered around 500 nm, representing the characteristic of CT states (Fig. 4a). Furthermore, to further verify the kinetic decay process of the exciplex excitons, the TrPL spectra (Fig. 4b and Fig. S21) confirm the successful formation of exciplexes, as evidenced by the clear signatures of delayed fluorescence. In the case of the PO-T2T : 2Cz2Cz (3 : 7) blend film, an additional lifetime component of 624.37 ns, indicating the presence of an extra relaxation pathway likely associated with an aggregation-related relaxation process, occurs in the excited-state kinetics at high 2Cz2Cz concentrations.³¹ According to the previous TADF kinetic studies,³² the equilibrium constant (K_eq = K_isc/K_risc) was obtained from the ratio of the pre-exponential factors, giving values of 5.964 and 5.427 for the PO-T2T:Cz2Cz and PO-T2T:2Cz2Cz blends, respectively. By applying the ΔE_ST − K_eq relationship, ΔE_ST = −RT ln(K_eq/3), where the factor of 3 accounts for triplet degeneracy, the ΔE_ST values were estimated to be 17.65 meV and 15.23 meV, respectively. Upon doping with the chiral helicene emitter, the emission spectra exhibited a significant bathochromic shift to 625 nm (Fig. 4a), confirming efficient energy transfer from the exciplex host to the emitter via FRET. This result substantiates the effective implementation of our host–guest design strategy. Interestingly, the TrPL spectra of the doped systems (Fig. 4c and Fig. S21) reveal three distinct lifetime components, suggesting the involvement of multiple emissive pathways. These pathways can be attributed to the complex photophysical interactions within the TADF-based exciplex co-host and fluorescent guest system. Specifically, the shortest lifetime corresponds to the prompt fluorescence of helicene emitter 3. The intermediate component is likely associated with aggregation of the emitter.⁴¹ The longest lifetime component listed in Table 3 is ascribed to the delayed fluorescence from the exciplex co-host following RISC. Notably, its τ_d is approximately one-tenth of that observed for the parent exciplex, likely due to the rapid depopulation of the singlet excited state via FRET.⁴² This behavior is consistent with previous observations in hyperfluorescent systems employing TADF hosts.^43,44

Fig. 4 (a) UV-vis Abs and Fl spectra of the exciplex and doped films at room temperature. (b and c) TrPL spectra of the doped system, exhibiting distinct FRET characteristics.

Table 3 Physical characteristics of Cz2Cz- and 2Cz2Cz-based exciplexes and 3-doped films

Blended film	λ Fl (nm)	PLQY (%)	τ PF (ns)	τ DF (ns)
PO-T2T : Cz2Cz (3 : 7)	487	26	41.22	2238.59
PO-T2T : 2Cz2Cz (3 : 7)	503	27	32.16/624.37	3125.13
PO-T2T : Cz2Cz : 3_P (3 : 7 : 1)	623	27	13.76/37.58	232.11
PO-T2T : Cz2Cz : 3_M (3 : 7 : 1)	625	30	21.02/47.45	349.31
PO-T2T : 2Cz2Cz : 3_P (3 : 7 : 1)	634	16	18.11/53.42	262.49
PO-T2T : 2Cz2Cz : 3_M (3 : 7 : 1)	634	16	15.79/52.51	262.06

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In addition, the photophysical behavior shows negligible variation between the P- and M-enantiomers of emitter 3. The absolute PLQYs measured using an integrating sphere indicate that the undoped PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) films exhibit PLQY of 26% and 27%, respectively (Table 3). Exciplex emission frequently exhibits low quantum yields due to the limited orbital overlap and subsequent small oscillator strength of the relevant transition, together with the possibility of exciton dissociation at the donor–acceptor interface.^26,34 In the PO-T2T : 2Cz2Cz : 3 (3 : 7 : 1) ternary system, pronounced 2Cz2Cz intermolecular interactions were identified, as confirmed by single-crystal XRD analysis, which revealed that the peripheral carbazole units promote intermolecular packing (Fig. S20b). This structural arrangement indicates a tendency toward partial aggregation rather than the formation of a fully amorphous co-host environment in the doped system, behavior that has also been observed in our previous studies.³⁴ As a result, the highly planar PDI moiety may not be sufficiently dispersed, leading to aggregation-induced quenching and a corresponding decrease in the PLQY. Interestingly, the enhanced aggregation in the PO-T2T:2Cz2Cz:3 system results in a red-shifted emission by ∼10 nm compared to that of the PO-T2T:Cz2Cz:3 system, as shown in Table 3. This behavior is likely to enhance the non-radiative decay channels, in accordance with the energy gap law, thereby resulting in a further reduction in PLQY.

The CPL properties of 3_P/M in toluene and 3-doped films were investigated (Fig. 5a–c). Upon excitation with unpolarized light, CPL spectra can be observed from both the pure CPL emitter 3_P/M and 3-doped films. The g_lum values are 5.1 × 10⁻⁴/−4.3 × 10⁻⁴ for 3 in toluene (Fig. 5a), and 6.8 × 10⁻⁴/−6.2 × 10⁻⁴ for PO-T2T:Cz2Cz:3 (Fig. 5b), and 6.9 × 10⁻⁴/−6.5 × 10⁻⁴ for PO-T2T:2Cz2Cz:3 (Fig. 5c). Notably, no significant difference was detected between the PO-T2T:Cz2Cz:3 and PO-T2T:2Cz2Cz:3 systems. This suggests that despite the different configurations of the two novel carbazole-derived donors, both exciplex-forming environments effectively facilitated FRET to the chiral emitter, resulting in the emission of CPL.

Fig. 5 g _lum Values of 3_P/M in (a) in toluene solution (10⁻⁵ M), and (b and c) in a doped film.

To investigate the potential of exciplex-forming systems as co-hosts for chiral emitters in CPL-OLED, devices with the structure of ITO/PEDOT:PSS/PO-T2T:(Cz2Cz or 2Cz2Cz)/PO-T2T (10 nm)/CN-T2T (40 nm)/LiF (1 nm)/Al (120 nm) were fabricated (Fig. 6a). The optimized conditions were found at an A : D ratio of 3 : 7, which yielded the best performance among the tested concentrations, as shown in Table S2. Due to the high molecular weight of 3, a solution-processing method was employed in this study. As depicted in Fig. 6b–f, the devices with the PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blends as the EML exhibit comparable device performances, with the EL centered at 506 nm and 503 nm, V_on of 4.8 V and 4.6 V, EQE_max of 6.38% and 7.30%, and maximum luminance reaching 4954 cd m⁻² and 4718 cd m⁻², respectively. Upon introducing chiral emitter 3, the CPL-OLEDs based on the PO-T2T : 2Cz2Cz : 3_P (3 : 7 : 1) system exhibit a remarkable g_lum of 6.9 × 10⁻⁴, demonstrating that the chiral dopant retains its excellent CPL characteristics even when embedded in the achiral exciplex co-host system (Table 4). In terms of device performance, the PO-T2T : Cz2Cz : 3 (3 : 7 : 1) system shows an EQE_max of 1.07% with EL emission centered at 616 nm, whereas the PO-T2T : 2Cz2Cz : 3 (3 : 7 : 1) device achieves a higher EQE_max of 1.38% with EL λ_max at 622 nm. Although the PLQY of the Cz2Cz-based system is higher (27%) than that (16%) of the 2Cz2Cz-based system, the deeper HOMO level (−5.66 eV) of Cz2Cz could have inferior hole injection capability, which leads to carrier accumulation. In contrast, the shallower HOMO level (−5.47 eV) of 2Cz2Cz facilitates better hole injection and carrier balance, thereby resulting in improved device efficiency despite its lower PLQY. In addition, the EL red-shifts significantly to approximately 620 nm, indicating efficient exciton energy transfer from the host to the chiral dopant (Fig. 6d). More importantly, this spectral shift is accompanied with clear CPL signals. Fig. 7 shows the CPEL spectra and g_EL values of the CPL-OLEDs based on the PO-T2T:Cz2Cz:3 and PO-T2T:2Cz2Cz:3 blends as the respective EML. The CPL-OLEDs showed clear symmetrical CPEL signals due to the configurational stability of 3_M/P. CPEL and EL were observed in the same emission wavelength range. The maximum g_EL values are 1.1 × 10⁻³ (P@660 nm)/−9.3 × 10⁻⁴ (M@660 nm) for PO-T2T:Cz2Cz:3, and 1.1 × 10⁻³ (P@651 nm)/−7.1 × 10⁻⁴ (M@651 nm) for PO-T2T:2Cz2Cz:3, respectively. The |g_EL| values of the CPL-OLEDs with 3_M/P as the chiral dopant are higher than that of most exciplex co-host systems reported for efficient CPL-OLEDs.^11,22 The superior device performance achieved by the material systems investigated in this work highlights their promising potential CPL-OLEDs, as referenced to previously reported devices with the CPL in the range of 600–700 nm (Table S3). Despite their similar EL wavelengths and chiroptical outputs, the PO-T2T:2Cz2Cz:3 system shows a more pronounced EQE roll-off, which may arise from the increased exciton annihilation or less favorable emitter dispersion within the co-host matrix. The rigid molecular conformation and strong intermolecular interactions of 2Cz2Cz (refer to Fig. S20) could hinder a uniform dopant distribution and exciton management under high current conditions. Nevertheless, efficient FRET still occurs in this system, as evidenced by the suppression of residual exciplex emission below 500 nm, which confirms that the emission originates predominantly from the dopant. These results demonstrate that exciplex co-host systems can not only serve as effective FRET mediators but also enable robust electrically driven CPL. This strategy offers a promising avenue toward high-efficiency CPL-OLEDs based on exciplex co-host systems accommodating a tailor-made helicene-based chiral dopant.

Fig. 6 (a) Illustration of the device structure with the energy levels, (b and c) current density (J) −luminance (L) −voltage (V) characteristics, (d) EL spectra of the exciplex and doped device, and (e and f) external quantum efficiency.

Table 4 EL performance of OLED devices

Device	PLpeak/ELpeak (nm)	V on (V)	EQEmax (%)	L max (cd m^−2)	PLQY (%)	g PL /g EL
PO-T2T : Cz2Cz (3 : 7)	497/506	4.8	6.38%@12.00 mA cm^−2	4954@8.8V	26	—
PO-T2T : 2Cz2Cz (3 : 7)	510/503	4.6	7.30%@2.56 mA cm^−2	4718@8.6V	27	—
PO-T2T : Cz2Cz : 3_P (3 : 7:1)	619/616	5.0	1.07%@13.91 mA cm^−2	1407@8.4V	27	6.8 × 10^−4/1.1 × 10^−3
PO-T2T : Cz2Cz : 3_M (3 : 7:1)	617/620	5.2	1.10%@12.37 mA cm^−2	1161@8V	30	−6.2 × 10^−4/−9.3 × 10^−4
PO-T2T : 2Cz2Cz : 3_P (3 : 7 : 1)	629/622	5.2	1.38%@2.47 mA cm^−2	977@9V	16	6.9 × 10^−4/1.1 × 10^−3
PO-T2T : 2Cz2Cz : 3_M (3 : 7 : 1)	626/626	5.4	1.41%@9.65 mA cm^−2	1109@9V	16	−6.5 × 10^−4/−7.1 × 10^−4

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Fig. 7 (a and b) CPEL spectra, and (c and d) g_EL values of CPL-OLEDs based on 3_M/P devices.

Conclusion

In summary, we have demonstrated a high-efficiency solution-processed CPL-OLED employing new exciplex-forming co-hosts doped with a helicene-based chiral emitter. Two highly twisted donor molecules, Cz2Cz and 2Cz2Cz, derived from the benchmark host material mCP were developed to combine with an acceptor PO-T2T for exploring exciplex formation, which was verified by the signature red-shifted emission compared to that of the components. The time-dependent photoluminescence supports these new exciplex-forming blends with TADF behavior, ensuring their high exciton harvesting capability. The devices employing the PO-T2T : Cz2Cz (3 : 7) and PO-T2T : 2Cz2Cz (3 : 7) blends as the EMLs exhibit EL λ_max at 506 nm and 503 nm, and EQE_max of 6.38% and 7.30%, respectively, which relevantly correlate to the observed PLQYs of 26–27%. The exciplex emissions nicely overlap with the absorption of the new chiral helicene emitter 3, ensuring the feasibility of adopting the host–guest strategy via efficient FRET. The chiral PDI-helicene emitter exhibits strong CPL signals (|g_PL| ≈ 6 × 10⁻⁴) and red emission in solution. The solution-processed red (626 nm) OLEDs utilizing the new exciplex-forming blends as the co-hosts of chiral emitter 3 gave the EQE_max up to 1.41% and the best |g_EL| ≈ 1.1 × 10⁻³. This work manifests a promising strategy for designing advanced CPL materials and devices, paving the way for the development of next-generation chiral display technology.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this study's findings are available from the corresponding author, upon reasonable request.

Supplementary information including synthetic procedure, spectral characterization, cyclic voltammogram, crystal data, theoretical calculation, time-resolved photoluminescence of blended films, and optical property of chiral emitter 3 is available. See DOI: https://doi.org/10.1039/d5tc02788h.

CCDC 2448238 contains the supplementary crystallographic data for this paper.⁴⁵

Acknowledgements

The authors thank the financial support from the National Science and Technology Council Taiwan (NSTC 112-2628-M-003-002-MY3, 112-2113-M-002-004, 112-2927-I-002-505, 114-2927-I-003-501, 114-2113-M-131-001-MY3, 113-2639-M-002-001-ASP). The authors further gratefully acknowledge the financial support from the European Commission (Grant Agreement No. 859752, HEL4CHIROLED-MSCA-ITN Project), Czech Scientific Foundation (Reg. No. 24-10787S), CAS (Reg. No. MOST-22-01), Praemium Academiae of the CAS 2025 (Reg. No. AP 2402) and IOCB CAS (RVO: 61388963). We are grateful to the National Center for High-performance Computing (NCHC) for providing computational and storage resources. We thank the Instrumentation Center, National Taiwan University (NTU), for the use of their facilities. We also thank the mass spectrometry technical research services from the NTU Consortia of Key Technologies for the assistance with matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy and electrospray ionization time-of-flight mass spectroscopy. We acknowledge the TGA data from the Thermal Analysis System of Instrumentation Center, NTU.

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